The invention relates to a receiver and a method for enhanced performance for CDMA transmission as described in the subiect of the independent claims.
In terrestrial communication the transmitted signal is reflected and refracted by a variety of smooth or rough terrains, so that it is replicated at the receiver with several time delays. Each individual path also arrives at its own amplitude and carrier phase. Propagation characteristics are qualitatively the same for all structures of signals though they will vary quantitatively with carrier frequency and terrain characteristics. The structures of the individual propagation paths can be identified and possibly exploited only to the extent that they can be distinguished from one other. In particular, spread spectrum signals employ pseudo-random sequences with a chip time Tc inversely proportional to the spreading bandwidth. In this case, the individual paths can be distinguished if they are mutually separated by delays greater than Tc, for then the various delayed versions of the signal will be mutually uncorrelated.
The situation is shown in FIG. 1. The path amplitudes (xcex1 will depend on the relative propagation distances and the reflective or refractive properties of the area. However, in many cases, particularly in confined areas, each of the distinguishable multipath components will actually be itself the linear combination of several indistinguishable paths of varying amplitudes. To exploit energy in the multiple components of multipath propagation, they must be identified and acquired. It is particularly important to determine the relative delays and subsequently, when possible, their amplitudes and phases. This can be performed even with fully modulated signals, but the estimate is much more precise and the resulting performance is much improved if the path identification and parameter estimation is performed on an unmodulated signal. Unmodulated segments can be inserted every so often in the modulated signal, particularly with time division multiplexing. However, in spread spectrum systems it is much more effective and easier to separate the unmodulated pilot signals from the data modulated signal by assigning it an individual pseudo-random sequence.
A pilot sequence for determining multipath component characteristics is well justified for one-to-many transmission channels such as the forward down link from a base station to multiple users.
The optimum demodulator structure for an L multipath propagation channel is known as Rake receiver. Each multipath component demodulator is called a xe2x80x9cfingerxe2x80x9d of the rake. The pilot sequence tracking of a particular demodulator is started by time delay estimation of a given path as determined by the pilot sequences searcher. The demodulator forms the weighted phase-adjusted and delay-adjusted sum of L components. In the prior art, the profile of the powers of each of the L paths is taken by checking the pilot sequence of one pilot channel on a slot-by-slot basis. This power profile is computed by non-coherent averaging of instantaneous channel profiles on this slot-by-slot basis. So the demodulator has to wait for the next pilot sequence in the next time slot to get more information to optimize the power profile.
For down link mode, the performance of this solution depends extremely on the signal-tonoise ratio of the pilot sequence. This means that the result of the demodulation in the receiver depends on the pilot sequence length itself and on the distortions in the dedicated channel.
Moreover, for high bitrates, the correlation length is shorter because of the lower spreading factor of the signal. Thus, for different bit rates, the performance of the path selection algorithm can be different.
The invention as described below increases performance of a demodulator in a CDMA receiver due to a method using more than one pilot sequence to extract good information about path delays and so to get an optimized channel profile.
The improvements are done by:
A receiver with enhanced performance for CDMA transmission comprising rake receiver with a path searcher (3), a channel estimator (4) and a combiner (5) for selecting optimal paths where the path searcher selects a set of path delays (7) with an instantaneous profile analyzer (8), an averoging filter (9) and a path selector (10), characterized in that at least two instantaneous profile analyzers (8) are connected to the averoging filter (9)
Or:
A receiver with enhanced performance for CDMA transmission comprising rake receiver with a path searcher (3), a channel estimator (4) and a combiner (5) for selecting optimal paths where the path searcher selects a set of path delays (7) with an instantaneous profile analyzer (8), on averaging filter (9) and a path selector (10), characterized in that at least two probes of pilot sequences are selected by a selection element (12) and are connected to one instantaneous profile analyzer (8) over a P-S converter (11).